EUTECTIC TRANSFORMATION IN DUCTILE CAST IRON. PART I THEORETICAL BACKGROUND

METALLURGY AND FOUNDRY ENGINEERING Vol. 31, 005, No. Edward Fraœ *, Marcin Górny **, Hugo F. López *** EUTECTIC TRANSFORMATION IN DUCTILE CAST IRON. PART I THEORETICAL BACKGROUND SYMBOLS Symbols Meaning Definition Units a Material mould ability to absorb of heat a = kmcm J/(cm ºC s 1/ ) A Parameter Eq. (10) s 1/ /m b Nucleation coefficient of graphite Eq. (58) o C B Temperature parameter Eq. (14) c Specific heat of metal Table 1 J/(cm 3 ºC) c m Specific heat of mould material J/(cm 3 ºC) C Carbon content in cast iron wt. % C e Carbon content in graphite eutectic Eq. () wt. % CT Chilling tendency Eqs. (67), (68) s 1/ / o C 1/3 C, C 3, C 4, C gr Carbon content in austenite, liquid and graphite Fig (3b) wt. % D Diffusivity of carbon in austenite Eq. (64) cm /s f gr Fraction of pre-eutectic graphite Eq. (1) * Prof., ** Ph.D., Faculty of Foundry Engineering, AGH University of Science and Technology, Cracow, Poland; edfras@agh.edu.pl; mgorny@agh.edu.pl, ** Author is an award holder of the NATO Science Fellowship Programme *** Prof., Department of Materials Engineering, University of Wisconsin-Milwaukee, P.O. Box 784, Milwaukee, WI 5301, USA; hlopez@uwm.edu 113

F c Surface area of the casting cm F ch Half surface area chill triangle Figure11 cm k Coefficient Eqs. (39), (43) k m Heat conductivity of the mould material J/(s cm ºC) l s l Size of substrate for nucleation of graphite Mean size of substrates for nucleation of graphite cm cm L e Latent heat of graphite eutectic Table 1 J/cm 3 m gr Mass of graphite Eq. (31) g m l Mass of graphite in liquid Eq. (30) g m γ Mass of graphite in austenite Eq. (35) g M Casting modulus Eqs. (11), (5), (55) cm M cr Critical casting modulus Eq. (70) cm N Volumetric nodule count Eqs. (57), (6) cm 3 N s Density of substrates for nucleation of graphite cm 3 P cr Coefficient Eq. (66) (cm o C 1/3 )/s 1/ P Phosphorus content in cast iron wt. % q a Accumulated heat flux in casting Eq. (6) J/s q m Heat flux extracted from casting into mould casting Eq. (5), (50) J/s q s Heat flux generated during solidification Eq. (7),(49) J/s Q Metal cooling rate Eq. (15) ºC/s R 1 Graphite nodule radius Figure 3b cm R Outer radius of austenite envelope Figure 3b cm R m Austenite envelope radius at maximum undercooling Eq. (47) cm s Wall thickness cm s cr Critical wall thickness Eqs. (71), (7) cm Si Silicon content in cast iron wt. % 114

t Time s t m Time at the maximum undercooling Eq. (1) s t s Time at the onset of graphite eutectic solidification s T Temperature ºC T c Equilibrium temperature of the cementite eutectic Table 1 ºC T i Initial temperature of metal in mould cavity ºC T m Temperature at maximum undercooling of graphite eutectic Figure a ºC T p Pouring temperature ºC T s Equilibrium temperature of the graphite eutectic Table 1 ºC V c Volume of casting cm 3 z Parameter Eq. (54) α Wedge angle Figure 11 o α d Thermal diffusivity of mould material k m /c m cm /s β Coefficient Table 1 o C 1 ΔH Enthalpy of metal J/cm 3 ΔΤ Undercooling of graphite eutectic ΔΤ m Maximum undercooling of graphite eutectic ΔΤ m = T s T m ºC ΔΤ sc Range of temperature, ΔΤ sc = T s T c Table 1 ºC θ Wetting angle between substrate and nuclei of graphite o ρ l Density of melt Table 1 g/cm 3 ρ gr Density of graphite Table 1 g/cm 3 ρ γ Density of austenite Table 1 g/cm 3 σ Interfacial energy between the nucleus and the melt J/cm ϕ, ϕ 1 Parameters Eqs. (5), (6) % ω Frequency Eq. (17) 1/s 115

1. INTRODUCTION Ductile cast iron is a modern engineering material whose production is continually increasing. Hence, there are numerous reports related to the production of nodular cast iron, some of which are of essential importance as they are linked to the solidification kinetics. In ductile cast iron, solidification is closely related to the nucleation and growth of nodular graphite. Yet, nucleation is the dominant process at the onset of solidification and it establishes the final nodule count. Accordingly, increasing the nodule count in a given cast iron leads to: Increasing strength and ductility in ductile cast iron [1]. Reduction of microsegregation of alloying elements [, 3], and improving microstructural homogeneity. Here, the type of eutectic transformation (stable or metastable) is also influenced due to the re-distribution of alloying elements. Reduction in the chilling tendency of cast iron [4, 5]. Increasing pre-shrinkage expansion [6]. Increasing formation of open and closed contraction cavity volumes [7]. Increasing fraction of ferrite in the microstructure [8]. The chilling tendency (CT) of cast iron dictates their subsequent performance in diverse applications. In particular, cast iron possessing a high chilling tendency is prone to develop zones of white or mottled iron. Considering that, these regions can be extremely hard, their machinability can be severely impaired. Alternatively, if white iron is the desired structure a relatively small chilling tendency favours the formation of grey iron. This in turn leads to poor hardness and wear properties for the cast components. Hence, considerable efforts have been made in correlating various factors of technological relevance (inoculation practice, iron composition, pouring temperature, etc.) with the chill of cast iron. In this work, a simple analytical model of eutectic transformation in ductile cast iron is proposed. The model enables the determination of nodule counts in nodular cast iron, the chilling tendency CT and chill of ductile cast iron.. ANALYSIS Ductile cast iron is often hypereutectic, where the volume fraction of pre-eutectic graphite f gr is given by fgr ( C C ) ρl e = 100 ρgr Ceρ l + C( ρl ρgr) (1) where Ce = 4.6 0.30Si 0.36P () The influence of carbon and silicon on the pre-eutectic volume fraction of graphite is shown in Figure 1. From this figure it can be observed that for a useful C and Si range in the 116

foundry practice f gr ranges from 0 to 0.0; as a result, solidification of pre-eutectic graphite can be neglected. In this case, during solidification, heat extraction leads to changes in the relative energies of solid and liquid in two ways: 1) a reduction in the enthalpy of the liquid or solid due to cooling given by Ä H = cdt, ) a release heat of the eutectic solidification through the enthalpy contribution, known also as the latent heat (L). Hence, the heat transfer process can be described by the heat balance equation Fig. 1. Effect of carbon and silicon on the volumetric fraction of pre-eutectic graphite in hypereutectic cast iron qm = qs + qa (3) Heat transfer during solidification can be very complex and analytical solutions are not always available, so numerical methods are commonly employed. However, in sand castings, heat transfer is mainly determined by the properties of the sand mold as shown by Chvorinov [9]. a) b) T i a) mould b) Temperature T s T m I II ΔT m Temperature metal T i t s t m Time Fig.. Cooling curve for eutectic ductile cast iron (a) and temperature profile in the mold-metal system (b) Figure b shows the ideal temperature profile expected for sand-mould metal systems. The heat flux going into the mold can be described by qm = Tkm ð á d t (4) 117

Taking into account the thermal diffusivity α d = k m /c m and the surface area F c of the casting, Eq. (4) can be rearranged as aft c qm = ð t (5) where a = k c. m m The above expression neglects curvature effects associated with the metal-mold interface. Moreover, it is assumed that the temperature distribution along the metal is uniform. Hence, the accumulated heat flux in the metal can be described by cvc dt qa = dt (6) The heat flux q s generated during solidification depends on the particular solidification mechanism and in the present work it can be described by dv qs = LeNVc dt (7) where dv/dt is the volumetric solidification rate of eutectic cells (cm 3 /s). During solidification, two main stages can be distinguished (Fig. a). First Stage In this stage, the temperature of the melt decreases from T i to the equilibrium temperature for graphite eutetctic T s with no solidification events (Fig. a). Accordingly, q s = 0 and Eq. (3) can be rewritten as q m = q a (8) Taken into consideration Eqs. (5) and (6) and solving Eq. (8) for the initial conditions: T = T i at t = 0, the temperature during this stage is given by t T = Ti exp AM where: A = c π a (9) (10) M V = F c c (11) 118

Differentiating Eq. (9) yields an equation for the cooling rate as dt dt T i t = exp AM t A M (1) For t = t s, T = T s, and the time elapsed during the first stage can be found from Eq. (9) t s = (A B M) (13) where B = T ln i Ts (14) The metal cooling rate at the end of first stage can be calculated by taking into account Eqs. (1) (14) for t = t s, T = T s condition and after rearranging, yielding 1) dt Ts a = Q = dt (15) π Bc M Second Stage During this stage, the solidification of graphite eutectic occurs in the t s < t < t m time range or in the T s < T < T m temperature range (Fig. a). The initial segment of the cooling curve where the eutectic transformation takes place can be defined as a function of the degree of maximum undercooling ΔT m = T s T m (Fig. a). The solidification temperature for this temperature range can be described by [10] ( ) T = Ts ΔTmsin ω t ts (16) where ω= π ( t t ) m s (17) Equation (16) is of empirical nature. However it has been shown to provide an excellent description of the experimental outcome [10]. Accordingly, during second stage the cooling rate can be found by differentiating Eq. (16). dt dt = ΔTmωcos[ ω( t ts)] (18) 1) It is assumed that cooling rate has positive sign. 119

Therefore at the beginning of stage II when t = t s, cos0 = 1, the cooling rate can be described by ) dt Q = =ω Δ Tm (19) dt Combining Eqs. (17) and (19), and rearranging, yields ΔTm ( t t ) m s = Q π (0) At the onset of eutectic solidification (t = t s ), the cooling rates in the first and second stages it can be assumed equal each other. Therefore, using Eqs. (13), (15), and (0), the time for the maximum undercooling can be determined from t m ( +π Δ ) π Bc M BTs T = 4 a T s m (1) Nuclei N of nodular graphite are formed in the casting volume of liquid V c, and from each nucleus rise a single eutectic cell of spherical shape. Hence, the volume of the eutectic 3 cell with radius R (Fig. 3b) is given by V = (4 π R ) /3, while the differential volume increment is given by dv = 4 π R dr. Accordingly, taking into account Eq. (7) the expression for the rate of heat generation can be described by 4 π Le N Vc R dr q s = dt () In order to determine the radius R, as well as the growth rate of the cell, dr /dt the eutectic transformation in nodular cast iron will now be considered. Taking into account diffusional transport during the eutectic transformation and assuming spherical geometry (Fig. 3), the steady state solution for carbon diffusion through the austenite shell can be defined by solving equation dc d C D + = 0 rdr dr (3) ) It is assumed that cooling rate has positive sign. 10

a) b) Temperatue T s J ' E B C D graphite austenite R 1 R T ΔT m S C C 3 C 4 Carbon content C gr C 4 C C 3 Fig. 3. Schematic representation of a Fe-C phase diagram section (a) and corresponding carbon profile for a spherical eutectic cell (b) The general solution to the above equation yields ϕ C =ϕ+ r 1 (4) where r is the radial coordinate, R 1 < r < R and ϕ and ϕ 1 are constants which can be determined from the boundary conditions: C = C 3 at r = R and C = C at r = R 1. This yields: R1(C C 3) ϕ= C3 R R 1 RR 1 (C C 3) ϕ 1 = R R 1 (5) (6) Hence, taking into account Eqs. (4), (6) the solute concentration profile in the austenite envelope can be described by: ( C C )( ) R1 3 r R C( r) = C3 r ( R R1) (7) Moreover, deriving Eq. (7) with respect to r yields the carbon concentration gradient in the austenite envelope at the liquid austenite interface. dc R1 R (C C 3) = dr r ( R1 R) (8) 11

Also, at the austenite solidification interface r = R, the concentration gradient becomes dc R1(C3 C ) = dr R R R ( ) 1 (9) Introducing a mass balance in the diffusional system (Fig. 3b) where (C 4 ) is the carbon content in the volume (4πR 3 )/3 prior to the eutectic transformation (when only liquid is the stable phase). In this case, the mass of carbon in the liquid can be given by ml 4 3 = π R C4 (30) 3 After the eutectic transformation, the diffusional system is depicted by a graphite nodule of radius R 1 and concentration C gr surrounded by an austenite envelope of radii R 1 and R. The mass of carbon in the graphite nodule can be determined from mgr 4 3 = π R1 Cgr (31) 3 A differential increment in the volume of the austenite envelope dv = 4πr dr causes a differential increment in the carbon mass according to dm γ = 4 π C( r) r dr (3) Also, the carbon mass in the austenite envelope is described by R R1 mγ = 4 π C() r r dr (33) Considering Eqs. (7) and (33) and rearranging terms yields 4 3 3 1 mγ = π C3R C R1 R1 R(C3 C )( R R1) 3 + (34) In the above equation, the third term within brackets is relatively small and can be neglected. This yields 3 3 ( C3 C 1 ) 4 mγ = R R 3 π (35) 1

From the mass balance (m l = m gr + m γ ), based on Eq. (30), (31) and (35) R 1 is found as 1/3 C4 C R 3 1 R = Cgr C (36) In addition, the condition for continuity across the liquid austenite interface needs to be taken into account. This condition using Fick law can be written as (C 4 dr dc C 3) D dt = dr (37) Combining Eqs. (9), (36), and (37) yields RdR = kddt (38) where k = ( ) (C C ) 3 C C gr C4 C3 3 1 C4 C3 (39) The above expression can be correlated with the degree of undercooling, ΔT. Assuming that the JE, E S and BC lines for the Fe-C system (Fig. 3a) are straight the concentrations in Eq. (39) can be given by: ΔT C = C E ' (40) m ΔT C3 = C E ' + (41) m 3 ΔT C4 = C C ' + (4) m 4 where: m, m 3 and m 4 are the slopes of the solubility lines JE, E S and BC respectively. 13

For Fe-C alloys the following values can be employed: C C = 4.6%, C E =.08%, m = 75 o C / %, m 3 = 189.6 o C / %, and m 4 = = 113. o C / %. Using this data, estimations of k values are plotted in Figure 4. From this figure, it is apparent that k tends to exhibit a linear trend with ΔT. Accordingly, k can be described by k = β ΔT (43) where β = 0.00155 o C 1. Moreover, it can be shown that the effect of Si on β is negligible. In addition, considering that the degree of undercooling ΔT = T s T, from Eqs. (16), (38) and (43), it is found that Fig. 4. Graphic plot of Eq. (39) function as a function of the degree of undercooling, ΔT ( ) R dr =βd ΔTmsin ω t ts dt (44) Hence, integrating the above expression for the limiting conditions t = t s at R = 0, yields β D ΔT R m = 1 cos ω t ts ω ( ) 1/ (45) Differentiating Eq. (45) with respect to time yields the rate of eutectic cell growth 1/ dr β D ωδt m = sin ω( t ts ) dt 1 cos ( t t ω s ) (46) Then, at the time t = t m, and by taking into account Eq. (17), expressions for the cell radius, as well as the cell growth rate at the maximum undercooling are found: 1/ β D ΔT R m, m = ω (47) 1/ dr, m β D ω ΔTm = dt (48) 14

In addition, by taking into account Eqs. (10), (13), (17), (1), (), (47) and (48), the heat flux generated during solidification at the time t = t m is given by cm qs = 4 LeN Vc a ( ) B ΔTm π β D ΔTm T s 3 1/ (49) Moreover, combining Eqs. (3) and (1) for the same time t = t m, the heat flux going into the mold can be described by 3/ Fc a T q s m = 1/ 1/ π cm B ( π Δ Tm + Ts B) (50) At t = t m, the cooling rate is zero and as a consequence the rate of heat accumulation q a is also zero. Hence, the heat balance equation (Eq. (3)) becomes qs = qm (51) Finally, by taking into account Eqs. (11), (1), (49) and (51) an expression for the modulus of casting can be obtained. M 1/3 a T s = 1/ 5/ 1/ ( Dβ) π BNLe c ΔTm ( π Δ Tm + TB s ) (5) In nodular cast iron, the pouring temperature, T p ranges from 150 to 1450 o C. Hence, taking into account that T p = T i, typical B values (Eq. (14)) range from 0.074 to 0.3. Figure 5 shows graphically the ( ) 1/ 1/ ΔTm π Δ Tm + TsB and z Δ TmTs functions versus ΔT m for two extreme values of B. From this figure it is evident that ( ) 1/ m m s s 1/ m ΔT π Δ T + T B z T Δ T (53) where z = 0.41 + 0.93B (54) Accordingly, taking into account Eqs. (5) and (53), N can be simplified to the following form M 1/ Ts 1 = a D 5/ β π zbnleδt m 1/3 (55) 15

a) b) m m s Δ π Δ + (solid line) and line) functions for: a) B = 0.074; b) B = 0.3 Fig. 5. Influence of ΔT m and B on the T ( T B T ) 1/ 1/ z Ts Δ Tm (dotted 3. NODULE COUNT Eq. (55) can be rewritten as a function of the cooling rate (taking into account Eq. (15)) as 1/ cb Q N = 1/ 4 π z L D e ΔT β m 3/ (56) The nucleation of graphite during solidification is of heterogeneous nature, on preferential substrates of various sizes, l s. The active substrate set is randomly distributed in the undercooled melt and it can be given by the substrate volume density, N s. A simple heterogeneous nucleation model previously developed [11] is employed in this work. Accordingly, the volume density of graphite nodules can be given by b N = Ns exp ΔT m (57) where 4 Ts σ sin θ b = Le l (58) Combining Eqs. (56) and (57) yields 1/ b c B Q Ns exp = ΔT π Δ D β 1/ m 4 z Le Tm 3/ (59) From this equation, the maximum degree of undercooling can be determined from Δ Tm = b ProductLog ( y) (60) 16

where 1/ 3 3 1/4 π zns Le D β y = b c 3 BQ (61) The ProductLog[y] = x is the Lambert function 3), also is called the omega function and it is graphically shown in Figure 6. This function is used to solve equation of the x type of y = xe. Its exact values can be easily calculated using the instruction of the ProductLog[y] in the Mathematica programme. In our analysis typical values of variable y are situated within boundaries from 0.03 to 500, and in case if we cannot Fig. 6. Graphic representation of the ProductLog[y] function for y 0 use Mathematica programme its value can be calculated by means of elementary mathematics with an accuracy higher than 99% using the following equations: for 0.03 y 1 range 0.3541y x = ; 0.03066y + 0.04476y + 0.35837 for 1 y 5 range 0.4539 0.94488 x= log(0.0855561) y + 3.03131 y ; for 5 y 0 range 0.00796 0.13581 x= log(0.06014) y + 3.9565 y ; for 0 y 50 range 0.04997 0.110107 x= log(0.038806) y + 3.5967 y ; for 0 y 500 0.34418 0.144835 x= log(0.05947) y +.03579 y. From Eqs. (57) and (60) N can be given by N N = s exp ProductLog( y) In metallographic determinations, the area density of graphite nodules N F is commonly measured at a given micro-section. In addition, in ductile iron the graphite nodules are cha- (6) 3) See http://mathworld.wolfram.com./lambertw-function.html. 17

racterized by Raleigh distributions [1]. In this case the volumetric nodule count (N) can be related to the area nodule count (N F ) using the Wiencek equation [13] ( ) 1/3 gr NF = f N (63) where f gr is the volume fraction of graphite in eutectic, f gr = 0.11. According to the literature, the nodule count N F is influenced by the chemical composition and the superheat temperature and time [14]. Yet, in the industrial practice, a drastic increase in nodule counts is usually obtained by the introduction of inoculants. In this case, the nodule count depends on the type of inoculant, quantity and granulation, Mg treatment and inoculation temperature, and the time after inoculation [14]. Also, it is well known that an increase in the cooling rates causes a radical increase in the nodule count in castings. From Eqs. (60) and (61) it can be seen that among the various factors that influence the final nodule count are: The diffusion coefficient of carbon in austenite, D depends on the actual temperature and chemical composition of the austenite. The effect of Si, Mn and P on D is not considered in this work as there is not enough information available in the literature. An expression for the diffusion coefficient of carbon in austenite has been published [15] 3.33957 1 D = 0.00453+ exp 15176.73 0.0001, cm /s T T (64) During the eutectic transformation, the temperature, T ranges from 1373.3 to 143.3 K, so the effective D values ranges from 3. 10 6 to 4.6 10 6 cm /s. Therefore, an average D value of 3.9 10 6 cm /s can be used without introducing significant error. In general, as D increases, the nodule count decreases as predicted by Eqs. (61) (11), (also see Fig. 7). Nucleation susceptibilities. Depending on the graphite nucleation susceptibilities in ductile cast iron at a given cooling rate, various nodule counts can be achieved. The nucleation susceptibilities of cast iron (Tab. 1) are characterized by N s and b. These coefficients (see Tab. 4, Part II) depend on the chemical composition, spheroidization practice, inoculation, and holding time of bath and temperature. Fig. 7. Influence of the diffusion coefficient for carbon in austenite on the nodule count; L e = 08.8 J/cm 3 ; c = 5.95 J/(cm 3 oc); β = 0.00155 o C 1 ; C = 3.71%, Si =.77%, P = 0.034%; T i = 1300 o C; D = = 3.9 10 6 cm /s (rest of data see Tab. 1) 18

Table 1. Physical-chemical data for nodular cast iron Latent heat of graphite eutectic L e = 08.8 J cm 3 Specific heat of the cast iron Coefficient of heat accumulation by the mould material c = 5.95 J cm 3 o C a = 0.1 J cm s 1/ o C Diffusion coefficient of carbon in austenite D = 3.9 10 6 cm s 1 Coefficient β = 0.00155 o C 1 Equilibrium temperature of the graphite eutectic Cementite eutectic formation temperature ΔT sc = T s T c T s = 1154 + 5.5Si 14.88P T c = 1130.56 + 4.06(C 3.33Si 1.58P), o C ΔT sc = 3.34 4.06C + 18.80Si +36.9P, o C Eutectic density ρ e = 0.96ρ γ + 0.074 ρ gr, g cm 3 Austenite density ρ γ = 7.51 g cm 3 Graphite density ρ gr =. g cm 3 Melt density ρ l = 7.1 g cm 3 Average temperature of the metal just after pouring in the mold cavity T i = 150 o C The influence of the nucleation coefficients on the nodule count is shown in Figure 8. The calculations made for nodule count were based on Eqs. (61) (63). From Figure 8, it is apparent that the nodule count increases when b decreases and N s increases. In addition, it is well known [11] that for given cooling rate prolonged bath holding time leads to reduction in N s and to an increase in b parameter. Consequently, prolonged bath holding time leads to a reduction in the nodule count. According to Eq. (15) the cooling rate increases as the ability of the mold to absorb heat a increases and as the casting modulus M decreases. The cooling rate is also dependent on the B parameter (Eq. 14), and hence on the initial temperature T i of the cast iron just after pouring into the mould. It also depends on the equilibrium eutectic temperature T s and the specific heat c (Eq. (15). Figure 9 shows the effect of B and of the a/m ratios on the cooling rate at the onset of eutectic solidification. A schematic diagram showing the role of various technological factors on the nodule count in ductile cast iron is given in Figure 10. 19

a) b) Fig. 8. Influence of the nucleation coefficient on the nodule count: a) N s = 5 10 7 cm 3 ; b) b = 60 o C (rest of data see Fig. 7) Fig. 9. Effect of B (T i, T s ) and a/m ratio on the cooling rate Q 130

Nodule count N Cooling rate Q Diffusion coefficient D Nucleation coefficients N s, b Initial temperature of melt T i Casting modulus M Mould material a Chemical composition of bath Furnace slag and atmosphere Spheroidizing and inoculation practice Temperature and holding time of bath Technological factors Fig. 10. Effect of technological factors on the nodule count in ductile cast iron 4. CHILLING TENDENCY Since ΔT m = T s T m, a plot of the function T m = f (Q) using Eqs. (60) and (61) is given in Figure 11. From this figure, it is found that the minimum temperature for the solidification of the graphite eutectic T m levels with the temperature T c when the cooling rate, Q = Q cr and ΔT m = T s T c = ΔT sc, so below T c, the solidification of cementite eutectic begins. By taking into account this condition into Eqs. (60) and (61) and using Eq. (15) an expression is derived for the critical casting modulus M cr = (V/F) cr below which a chill is likely to develop in castings Mcr = pcr CT (65) where the coefficient p cr includes the parameters (a, z, B T s ) related to the cast iron cooling rates 4) p cr 3 T s = a 5 4 4 π β z B Le c 1/6 (66) 4) Influence of L e and c can be neglected. 131

Fig. 11. Influence of the cooling rate Q on the minimal solidification temperature T m for graphite eutectic (rest of data see Fig. 7) CT is the chilling tendency for ductile cast iron 1 1 b CT = exp 1/ D N T s βδt Δ sc sc 1/3 (67) From Eqs. (57) and (67), for ΔT m = ΔT sc, CT can be simplified to 1/3 1 1 CT = 1/ D N T βδ sc (68) Typical CT values range from 0.8 to 1. s 1/ / o C 1/3. The influence of the various terms on CT, (Eq. (68)) can be summarized as: Diffusion coefficient, as D increases, CT decreases. The temperature range ΔT sc = T s T c, depends on the chemical composition as o Δ = 3.34 4.07C + 18.80Si + 36.9P, C (69) T sc In particular, carbon reduces ΔT sc while silicon expends it (Fig. 1a). The effect of this range on CT is shown in Figure 1b. Notice from this figure that CT decreases with increasing ΔT sc ranges. The Nucleation susceptibilities of graphite depends on the resultant values of N s and b (see previous section). In particular, CT increases with increasing b and decreasing N s (Fig. 13). 13

a) b) Fig. 1. Influence of carbon and silicon on: a) ΔT sc ; b) ΔT sc on the chilling tendency CT Fig. 13. Influence of N s and b on the chilling tendency of ductile cast iron; ΔT sc = 60 o C (rest of data see Fig. 7) 5. CHILL In establishing the chill of cast iron, it is common to implement a set of chill tests on castings, which consist of plates with different wall thicknesses, thus the critical casting modulus can be determined from M cr V = F cr cr (70) where V cr and F cr are chilled volume and surface area of casting. 133

Clill s cr Influence on cooling rate Temperature range ΔT ec Chilling tendency CT Diffusion coefficient D Nodule count N Nucleation parameters N s, b Initial temperature of melt T i Mould Material a Eutecic temperature T e Chemical composition of bath Furnace slag and atmosphere Spheroidizing and inoculation practice Temperature and holding time of bath Technological factors Fig. 14. Effect of technological factors on the chill and chilling tendency of ductile iron For plates with wall thickness, s and lengths and widths which easily exceed s, M cr = = V cr /F cr = (s cr F cr )/( F cr ), and the critical wall thickness s cr below which the chill forms is scr = Mcr (71) and after using Eq. (65) scr = pcr CT (7) From Eqs. (66), (67) and (7) results a diagram showing the influence of technological factors on chilling tendency CT and the chill of ductile cast iron is shown in Figure 14. 6. CONCLUSIONS A theoretical analysis is proposed for the solidification of ductile cast iron which allows to predict nodule counts based only on the knowledge of cooling rates and chemical composition, the initial temperature of the metal just after pouring into the mould, the diffu- 134

sion coefficient of carbon in austenite and the graphite nucleation coefficients. From the proposed theory, expressions are derived for the chilling tendency and chill of cast iron. A summary of the present analysis on the nodule count of cast iron indicates that it depends on: The chemical composition of the cast, through D, N s, and b. The inoculation practice, the bath superheating temperature and time, furnace slag and atmosphere, through N s and b. Cooling rate, through M, a and B. The chilling tendency CT of the nodular cast iron depends on: Chemical composition through D, ΔT sc, N s, and b, Inoculation and spheroidization practice, superheat temperature and time, as well as furnace slag and atmosphere through N s and b. In addition, the chill of the nodular cast iron is related to cooling rate Q and CT. REFERENCES [1] Jincheng Liu, Elliot R.: The influence of cast structure on the austempering of ductile iron. International Journal of cast Metals Researches, 11 (1999), p. 407 [] Lesoult G., Castro M., Lacaze J.: Solidification of Spheroidal Graphite Cast Iron: III Microsegregation related effects. Acta Materialia, 47 (1999), p. 3779 [3] Achmadabadi M.N., Niyama E., Ohide T.: Structural control of 1% Mn aided by modeling of micro-segregation. Transactions of the American Foundrymans Society, 97 (1994), p. 69 [4] Javaid A., Thompson J., Davis K.G.: Critical conditions for obtaining carbide free microstrucutre in thinwall ductile iron. AFS Transactions, 10 (00), p. 889 [5] Javaid A., Thompson J., Sahoo M., Davis K.G.: Factors affecting formation of carbides in thin-wll DI castings, AFS Transactions, 99 (1999), p. 441 [6] Fraœ E., Lopez H.: Generation of International Pressure During Solidification of Eutectic Cast Iron. Transactions of the American Foundryman Society, 10 (1994), p. 597 [7] Fraœ E., Podrzucki C.: Inoculated Cast Iron. AGH, Cracov, 1981 [8] Venugopalan D.: A Kinetic Model of the γ α + Gr Eutectoid Transformation in Spheroidal Graphite Cast Iron. Metallurgical and Materials Transactions, 1 A (1990), p. 913 [9] Chvorinov N.: Theorie der Erstarrung von Gusstûcken. Die Giesserei (1940) 7, p. 10 [10] Fras E., Lopez H.: A theoretical analysis of the chilling suspectibility of hypoeutectic Fe-C alloys. Acta Metallurgica and Materialia, 41 (1993) 1, p. 3575 [11] Fras E., Górny M., Lopez H., Tartera J.: Nucleation and grains density in grey cast iron. A theoretical model and experimental verification. International Journal of Cast Metals Research, 16 (003), pp. 99 104 [1] Wojnar L.: Effect of graphite size and distribution on fracture and fractography of ferritic nodular cast iron. Acta Stereologica, 5/ (1986), p. 319 135

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